It is known that most airliners are equipped with anticollision systems (generally called TCAS systems for Traffic Collision Avoidance Systems) which make it possible to ensure the safety of air traffic by preventing the risks of in-flight collision.
Thus, when two airplanes are converging towards one another, their anticollision systems calculate an estimate of the collision time and emit an alert informing the crews of each airplane of a possible future collision: such an alert is generally called a “traffic advisory” or “TA alert”. If appropriate, said anticollision systems emit moreover, for the attention of the crew, an order regarding an avoidance maneuver in the vertical plane so as to get out of the situation in which a collision is possible: such an avoidance maneuver order is generally called a “resolution advisory” or “RA alert”. The TA and RA alerts are manifested through voice messages and through the displaying of information in flight cabins.
In practice, an onboard anticollision system calculates a collision time in the horizontal plane (ratio of the horizontal distance of the two airplanes to their relative horizontal speed) and a collision time in the vertical plane (ratio of the vertical distance of the two airplanes to their relative vertical speed). Said collision times thus calculated are compared with predetermined thresholds for the TA alerts and for the RA alerts (said predetermined thresholds being moreover dependent on the altitude) and said alerts are triggered when said calculated collision times are less than the corresponding predetermined thresholds.
Moreover, it is known that frequently an airplane has to capture (while climbing or descending) a stabilized altitude level neighboring another altitude level allocated to another airplane and that, according to the rules of aerial navigation, two neighboring stabilized altitude levels are separated by only 300 m (1000 feet).
Hence, because of this small difference in altitude between stabilized altitude levels, the high vertical speed of modern airplanes and the weight of air traffic, said anticollision systems produce numerous TA and RA alerts, even though the airplane, shifting vertically so as to change altitude, is maneuvering correctly without any risk of collision with another airplane. These alerts induce a great deal of stress and are deemed operationally unnecessary by pilots, since the change-of-altitude maneuver is correct and their consideration leads to traffic disruption in most cases.
Moreover, the RA alerts during the altitude capture phases are very numerous and it is estimated that they currently represent more than 50% of the total of these alerts in European space, this percentage being apt to increase in the future owing to the expansion of air traffic.